1. Field of the Invention
The present invention relates generally to evaporative emission control in motor vehicles. More particularly, the invention relates to a diagnostic method for monitoring an evaporative emission system of a motor vehicle.
2. Background Art
Conventional motor vehicles are well known to release evaporative hydrocarbons into the atmosphere during both operating and non-operating states of the vehicle. Consequently, laws and regulations have been established requiring on-board vehicle evaporative emission systems to control the amount of fuel vapors emitted into the atmosphere. Such systems typically include a carbon filled canister and one or more valves for collecting, routing and venting unburned hydrocarbon emissions.
To monitor the level of hydrocarbon emissions from such systems, so-called On-Board Diagnostics (OBD) systems are used to insure that a vehicle""s evaporative emission system and powertrain components are operating in compliance with government standards. Conventional diagnostic systems, including OBD systems, utilize pressure or vacuum tests to monitor hydrocarbon emissions. Generally, these systems apply a partial vacuum to the fuel tank of the vehicle until a predetermined pressure level is reached. Once the predetermined pressure level is reached, the tank is sealed and the system measures the amount of vacuum xe2x80x9cbleed offxe2x80x9d over a predetermined period of time. An example of one such diagnostic system is described in U.S. Pat. No. 5,261,379 to Lipinski et al., which is also owned by the assignee of the present application.
Conventional diagnostic systems require that diagnostic tests be performed while the vehicle is running and in an operative state. Consequently, changing environmental and operating conditions tend to affect a system""s detection of low-level hydrocarbon emissions. Significant factors that may be considered include fuel xe2x80x9csloshingxe2x80x9d, changes in fuel temperature and barometric pressure, heat introduced by circulated fuel, fuel evaporative characteristics, tank flex, the age of the fuel, and ambient or underbody air temperature.
Fuel sloshing can occur during idle conditions (fuel circulation due to fuel pump), steady state operation (small agitation), or during stopping, starting and braking conditions (large agitation). As the fuel sloshes within the tank, the chemical reactions that produce fuel vapor occur at a faster rate thus increasing the gas volume and pressure inside the fuel tank. Also, as cooler xe2x80x9csloshingxe2x80x9d liquid fuel comes into contact with warmer tank surfaces, the resulting temperature differential enhances the rate of fuel vaporization and thus fuel tank pressure. Consequently, since typical fuel tank pressures are measured on the order of inches of water, even the smallest changes in fuel tank pressure can influence the results of an emissions detection evaluation.
Another factor relates to the effect of external pressure changes on the pressure sensors used in conventional systems. Because conventional systems use sensitive gage pressure sensors that measure differences between a pressure/vacuum source and a reference source, typically the atmosphere, such systems are susceptible to small variations in fuel vapor pressure attributable to movement of the vehicle. Normal changes in atmospheric pressure are approximately equal to one inch of mercury per 1000 feet of elevation, or 1.36 in of water per 100 feet of elevation. For example, if a vehicle were traveling up or down a hill with a 5% grade at 60 mile per hour, the elevation would change 264 feet every 60 seconds and thus the atmospheric reference pressure would change by 3.59 inches of water every 60 seconds. Therefore, if the atmospheric reference source changes so does the relative measurement of the pressure/vacuum source.
Other limiting factors include fuel temperature effects related to the proximity of the fuel tank to the vehicle""s exhaust system and whether the vehicle has a so-called xe2x80x9creturnxe2x80x9d or xe2x80x9creturnlessxe2x80x9d fuel system.
A method is provided for monitoring evaporative emissions from a vehicle having an evaporative emission system, the method including the steps of: establishing a baseline vapor pressure within the evaporative emission system during a vehicle-off; detecting a change in the baseline vapor pressure during the vehicle-off condition; and indicating whether the change in the baseline vapor pressure is within a desirable limit. In accordance with the present invention, a xe2x80x9cvehicle-offxe2x80x9d condition is defined as a mode wherein the vehicle is stationary with the internal combustion engine turned off. To implement the method, a corresponding system is also disclosed, the system having a sensor for monitoring a baseline vapor pressure of the evaporative emission system and subsequent changes thereto occurring during a vehicle-off condition, and a controller activateable during the vehicle-off condition and coupled to the evaporative emission system and the sensor for indicating whether the change in the baseline vapor pressure is within a desirable limit.
An advantage of the above-described diagnostic method and corresponding system is that vehicle evaporative emissions can be detected while the vehicle is in an engine-off condition, thereby substantially eliminating sources of undesired pressure disturbances, such as fuel slosh, temperature gradients, barometric pressure changes, etc. By performing a diagnostic test while the engine is off and after it has been stationary for a predetermined period of time, pressure disturbances are significantly reduced thereby enabling accurate and repeatable detection of lower-level hydrocarbon emissions.
Further objects, features and advantages of the invention will become apparent from the following detailed description of the invention taken in conjunction with the accompanying figures showing illustrative embodiments of the invention.